The use of bulky ligand transition metal metallocene-type catalyst systems in polymerization processes to produce a diverse array of new polymers for use in a wide variety of applications and products is well known in the art. Typical bulky ligand transition metal metallocene-type compounds are generally described as containing one or more ligands capable of η-5 bonding to a transition metal atom, usually, cyclopentadienyl derived ligands or moieties, in combination with a transition metal selected from Group 4, 5 or 6 or from the lanthanide and actinide series of the Periodic Table of Elements. Exemplary of the development of these and other metallocene-type catalyst compounds and catalyst systems are described in U.S. Pat. Nos. 5,017,714, 5,055,438, 5,096,867, 5,198,401, 5,229,478, 5,264,405, 5,278,119, 5,324,800, 5,384,299, 5,408,017, 5,491,207 and 5,621,126 all of which are herein fully incorporated by reference.
Commercial scale polymerization processes that typically operate with a recycle system have had problems with reactor operability and maintaining consistent catalyst productivities. Commercial scale polymerization processes are distinct from most small or laboratory sized operations. Some industrial reagents or feed differ in purity from those typically used at the commercial level and often vary from supplier. The size of the commercial reactors and duration of the runs makes it more difficult to control various conditions and the like compared with a small laboratory reactor operating for a few hours or less. Generally, in commercial polymerization processes reactants and components entering the reactor are withdrawn from the reactor and are recycled back to the reactor. This recycle system is typically not something found on a small laboratory scale reactor. Also, commercial production usually is continuous and production extends over days or weeks. Thus, in a small scale operation upsets are not easily discoverable.
It is also well known that these metallocene-type catalyst systems have a tendency toward fouling and/or sheeting, particularly in a commercial process. For example, in a continuous slurry process fouling on the walls of the reactor, which acts as heat transfer surface, can result in many problems. Poor heat transfer during polymerization can result in polymer particles adhering to the walls of the reactor, where they can continue to polymerize. This can be detrimental to the process and can result in premature reactor shutdown. Also, depending upon the reactor conditions, some of the polymer may dissolve in the reactor diluent and redeposit on for example the metal heat exchanger surfaces.
In a continuous gas phase process for example, a continuous recycle stream is employed. The recycle stream composition is heated by the heat of polymerization, and in another part of the cycle, heat is removed by a cooling system external to the reactor. Fouling in a continuous gas phase process can lead to the ineffective operation of various reactor systems. For example, the cooling system, temperature probes and the distributor plate, which are often employed in a gas phase fluidized bed polymerization process can be affected. These upsets can lead to an early reactor shutdown.
As a result of the reactor operability issues associated with using bulky ligand transition metal metallocene-type catalysts and catalyst systems various techniques have been developed that are said to result in improved operability. Improved operability would include better catalyst productivity, improved polymer products and extended operation.
For example, various supporting procedures or methods for producing a metallocene-type catalyst system with reduced tendencies for fouling and better operability have been discussed in the art. U.S. Pat. No. 5,283,218 is directed towards the prepolymerization of a metallocene catalyst. U.S. Pat. Nos. 5,332,706 and 5,473,028 have resorted to a particular technique for forming a catalyst by “incipient impregnation”. U.S. Pat. Nos. 5,427,991 and 5,643,847 describe the chemical bonding of non-coordinating anionic activators to supports. U.S. Pat. No. 5,492,975 discusses polymer bound metallocene-type catalyst systems. U.S. Pat. No. 5,661,095 discusses supporting a metallocene-type catalyst on a copolymer of an olefin and an unsaturated silane. PCT publication WO 97/06186 published Feb. 20, 1997 teaches removing inorganic and organic impurities after formation of the metallocene-type catalyst itself. PCT publication WO 97/15602 published May 1, 1997 discusses readily supportable metal complexes. PCT publication WO 97/27224 published Jul. 31, 1997 relates to forming a supported transition metal compound in the presence of an unsaturated organic compound having at least one terminal double bond.
Others have discussed different process modifications for improving operability. For example, PCT publication WO 97/14721 published Apr. 24, 1997 discusses the suppression of fines that can cause sheeting by adding an inert hydrocarbon to the process. U.S. Pat. No. 5,627,243 discusses a new type of distributor plate for use in fluidized bed gas phase reactors. PCT publication WO 96/08520 discusses avoiding the introduction of a scavenger into the reactor. U.S. Pat. No. 5,461,123 discusses using sound waves to reduce sheeting. EP 0 453 116 A1 published Oct. 23, 1991 discusses the introduction of antistatic agents for reducing the amount of sheets and agglomerates. U.S. Pat. No. 5,066,736 discusses the introduction of an activity retarder to the process to reduce agglomerates. U.S. Pat. No. 5,610,244 relates to feeding make-up monomer directly into the reactor above the bed to avoid fouling and improve polymer quality. U.S. Pat. No. 5,126,414 discusses including an oligomer removal system for reducing distributor plate fouling and providing for polymers free of gels.
There are various other known methods for improving operability including coating the polymerization equipment, injecting various agents into the reactor, controlling the polymerization rate, particularly on start-up, and reconfiguring the reactor design.
While all these possible solutions might reduce fouling or sheeting somewhat, some are expensive to employ and/or may not reduce both fouling and sheeting to a level sufficient for the successful operation of a continuous process, particularly in a commercial or large-scale process.
Thus, it would be advantageous to have a polymerization process capable of operating continuously, commercially, with enhanced reactor operability while at the same time producing polymers having improved physical properties. It would also be highly advantageous to have a continuously operating polymerization process having more stable catalyst productivities and reduced fouling/sheeting tendencies and increased duration of operation.